Scientists are one step closer to cracking the cosmic case of a dwarf planet's disappearing craters

NASA/JPL-Caltech/SwRI Ceres is a 4.5 billion-year-old dwarf planet living in the clamorous asteroid belt between Mars and Jupiter. It's the largest object in this asteroid belt and throughout its long life, it should have been relentlessly pummeled by large space rocks, leaving it pockmarked and scarred with craters.

And yet, when NASA's Dawn spacecraft arrived at Ceres in March 2015, scientists found that the dwarf planet was surprisingly smooth. For some reason, some of the biggest craters had been leveled.

This left scientists with a huge cosmic mystery: What happened to Ceres' missing craters?

The mystery of the missing craters

Based on collision models, scientists figured Ceres should have accumulated about 10 to 15 craters larger than 250 miles wide, and at least 40 craters larger than 62 miles wide. But, while Ceres is covered in small, young craters, scientists only found 16 craters larger than 62 miles, and none larger than 175 miles across.

With Dawn's previous subject, a protoplanet called Vesta that is about half the size of Ceres, scientists had found a handful of large craters, including a well-preserved crater about 300 miles wide. And yet the biggest crater on Ceres was less than 200 miles in diameter.

What they did find in the place of craters, though, were a few 500-mile wide depressions, which scientists believe could be ancient impact craters that are still being filled in and marked with smaller, more recent craters.

So what happened?

The team of scientists suggest that Ceres' icy, possibly salty interior might weaken its crust, allowing its surface to refresh itself.

In an Arizona State University press release, David Williams, co-author of the study, said that Ceres must also have generated some internal heat from the decay of radioactive elements after it formed, and this heat might have played a hand in softening or erasing the largest craters.

Ceres' geological history might also be to blame: Giant ice- and water-spewing volcanoes might have leveled the largest craters.

As Ceres makes its closest approach to the Sun in April 2018, NASA hopes Dawn will continue to orbit, investigating if the sun's warmth causes any changes in the surface of the dwarf planet.

"Ceres is revealing only slowly the answers to her many mysteries," Williams said. "Completing the geological maps over the next year, and further analysis of the compositional and gravity data, will help us understand better Ceres' geologic evolution."